The present invention relates to wireless communication systems. More particularly, and not by way of limitation, the present invention is directed to a transmitter, receiver, and method for channel estimation for a Multiple-Input Multiple-Output (MIMO) communication system.
Recent advances in wireless communications have caused a revolution in the Internet by extending broadband coverage to wireless users. Table 1 below is a classification of past, present, and future cellular technologies. It is seen that wireless cellular technologies have progressively and systematically raised their performance levels by an order of magnitude over previous generations. Spectral efficiency and data rate are key metrics that have improved. Some of this improvement has come about due to increases in the allocated spectrum, while other improvements are the result of technological advances, the most important of which have been the introduction of flexible and adaptive channel coding and modulation, dynamic link adaptation to choose the best data rate for the radio channel conditions, and the introduction of Multiple-Input Multiple-Output (MIMO) systems that utilize multiple antennas at the transmitter and or receiver to improve the number of degrees of freedom available to reach the users (or to receive from them). Similar advances have also been made by standards developed by the Institute of Electrical and Electronics Engineers (IEEE) within the Wireless Local Area Network (WLAN) standard IEEE 802.11, as further developed by the WiFi alliance.
TABLE 1ITU-R Classification—IMT-2000IMT-Advanced1G2G3G4GAMPS/NMTGSM/EDGEGSM/WCDMA/3GPP LTEEIA/TIA-136HSPA/EDGERel. 10EIA/TIA-95CDMA2000/evDO3GPP LTE Rel. 8AnalogDigitalDigitalDigital<100 KHz<1 MHz<10 MHz<100 MHz400-1000 MHz400-2000 MHz400-3000 MHz200-5000 MHz<10 kb/s/user<1 Mb/s/cell<100 Mb/s/cell<1 Gb/s/cellVoiceVoice/dataVoice/dataData (voicetelephonyincluded)1985-19991992-present2002-present2009-present
MIMO antenna systems increase the throughput of a wireless communication link without bandwidth expansion. Through proper design of the signal-transmission scheme and the corresponding receiver algorithm, the MIMO channel may be decomposed into parallel non-interfering channels. The number of such parallel channels, or streams, is the smaller of the number of transmit antennas and the number of receive antennas. When all the receive antennas are attached to the same User Equipment (UE), the system is referred to as Single-User MIMO (SU-MIMO). When the receive antennas are distributed among multiple UEs, the system is referred to as Multi-User MIMO (MU-MIMO). Because of its great potential in bandwidth efficiency, MIMO has been adopted by most wireless communication standards such as Long Term Evolution (LTE), WiMax, and WiFi.
In order to exploit the potential of the MIMO channel, it is critical to have knowledge of the MIMO channel state information. This information is needed for the receiver to perform the demodulation of transmitted data symbols. It is also needed sometimes at the transmitter to properly shape the transmit signal to improve Signal to Interference and Noise Ratio (SINR) at the receiver.
In a wireless communication system employing Orthogonal Frequency Division Multiplexing (OFDM), the channel state information can be modeled as a slowly varying, 2-dimensional, complex time-frequency process. Known Reference Signals (RS), i.e, pilot symbols, are transmitted at various time instants and frequencies.
FIG. 1 illustrates a pilot RS for MIMO in an LTE system. When the RSs are properly distributed across the time-frequency plane, the receiver can use these known symbols to reconstruct the full channel response. Naturally, the density of the pilots depends on the rate at which the channel varies in time and frequency. For LTE employing two transmit antennas, the pilots are transmitted at a higher density, as shown in FIG. 1. For four transmit antennas, the last two antennas have lower pilot density since it is anticipated that the channel variation is slower for the scenarios that can exploit four antennas.
Since the antennas are co-located, the transmit signals interfere severely with each other. In order for the receiver to receive the pilots without interference, an antenna mutes its transmission at the locations where pilots are transmitted by other antennas, as marked by the shaded areas. In total, 24 resource elements out of every 168 are reserved overhead for pilot transmission in LTE.
Recently, there have been growing interests in extending the MIMO system to a very large number of transmit and receive antennas. Instead of four to eight antennas typically employed in current systems, the number of antennas envisaged in these recent studies ranges in the order of 100 or more. Supported by the random matrix theory, it has been suggested that the required energy per bit vanishes as the number of antennas goes to infinity.